In previously-proposed data storage media such as hard disk-drives or optical disk drives, information is encoded magnetically, optically or a combination thereof onto a disk. This information is retrieved by detecting a signal corresponding to the method used for encoding the information, that is, by detecting magnetic stray fields, changes of optical reflectivity or a combination thereof, respectively. Due to the nature of the signals to be detected, sophisticated detectors are used. Thus, such data storage media may not be suitable for use with probe-type data storage devices.
A probe-type data storage device based on the atomic force microscope (AFM) is disclosed in “The millipede—more than 1,000 tips for future AFM data storage” by P. Vettiger et al., IBM Journal Research Development, Vol. 44, No. 3, March 2000. The storage device has a read and write function based on a mechanical x-, y-scanning of a storage medium with an array of probes each having a tip. The probes operate in parallel with each probe scanning, during operation, an associated field of the storage medium. The storage medium comprises a polymethylmethacrylate (PMMA) layer. The tips, which each have a diameter of between 5 nm to 40 nm, are moved across the surface of the polymer layer in a contact mode. The contact mode is achieved by applying forces to the probes so that the tips of the probes can touch the surface of the polymer layer. For this purpose, the probes comprise cantilevers, which carry the tips on their end sections. Bits are represented by indentation marks, each encoding a logical “1”, or non-indentation marks, each encoding a logical “0”, in the polymer layer. The cantilevers respond to these topographic changes while they are moved across the surface of the polymer layer during operation of the device in read/write mode.
Indentation marks are formed on the polymer layer by thermomechanical recording. This is achieved by heating the tip of a respective probe operated in contact mode with respect to the polymer layer. Heating of the tip is achieved via a heater dedicated to the writing/formation of the indentation marks. The polymer layer softens locally where it is contacted by the heated tip. The result is an indentation mark, for example, having a nanoscale diameter comparable to the diameter of the tip that is used in its formation, being produced on the layer. Reading of the indentation mark is also accomplished by a thermomechanical concept and may be done using the same probe as that used for writing the indentation mark. Due to the mechanical stress that is used for writing indentation marks in the polymer layer, tip and/or media wear may be typically expected to occur.
In another previously-proposed probe-type data storage device as described in Ultramicroscopy, 42-44 (1992) 262, data is encoded on an insulator such as Nitride-Silicon Dioxide-Silicon by charge injection, that is, bits are represented by localized trapped charges on the insulator surface. Thus, each trapped charge denotes a logical “1” and the absence thereof denotes a logical “0”. Data is retrieved by detecting the electrical stray field associated to each of the localized trapped charges, which field gives rise to a measurable but relatively small interaction force that is on the order of, for example, 1 nN. Other issues that may need to be considered in the detection of the localized trapped charges are: (1) the aforementioned electrical stray field is long range by nature and so may result in the “smearing out” of a bit location; (2) a localized trapped charge is typically screened by polar contaminants, for example, water molecules, thereby reducing the magnitude of an associated electrical stray field by an order of magnitude within a short time, typically within 24 hours, of charge injection, and (3) the magnitude of the aforementioned interaction force may limit data rates on the order of kHz rather than MHz
In yet another previously-proposed probe-type data storage device, bits are stored as oriented domains in ferroelectric media analogous to magnetic recording. Detection of an electric dipole orientation associated to a domain may be performed by measuring the strength of a corresponding electrical stray field. However, the issues listed under (1) to (3) above may also need to be considered in the present case. Alternatively, detection of the electric dipole orientation may be done by measuring the piezo-electric response, which induces minute, well-localized modulations of the surface topography on the order of a fraction of a nanometer, which requires sensitive lock-in schemes for their detection. In this case, it could be that signal degradation may be avoided by the electromechanical transduction. However, by virtue of being on the order of 0.1 nm, the topographic features may be comparable to or less than the roughness of the surface on which they are present. Furthermore, the detection of such sub-nanometer dimensioned features using known detectors may typically be done with a limited data rate in the kHz range.
Accordingly, it is desirable to provide a data storage device having topographic features that may be detected with relative ease compared to previously-proposed devices.